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General description

Vitamin D is a fat-soluble vitamin that is necessary for bone growth and development. Because it is stored by the body, taking too much vitamin D can cause toxicity problems.

Vitamin D deficiency may result in rickets (a disease affecting the skeletal system).

Ergosterol (provitamin D-2) and ergocalciferol (vitamin D-2) are found in plants; 7-dehydrocholesterol is found in animals. Vitamin D is called the sunshine vitamin because in humans the biologically active form, calciferol (vitamin D-3) is formed after exposure to sunlight.

Vitamin D, in conjunction with the parathyroid hormone, regulates calcium balance in the body and maintains normal serum calcium levels and bone density. Either formed through the exposure of sunshine on the skin or obtained through the diet, vitamin D increases the absorption of calcium and phosphorus from the intestine and aids in the reabsorption of phosphorus from the kidney. Vitamin D is essential for normal bone growth and for repairing bones after a fracture. As a result, vitamin D plays an essential role in the growth and development of the skeletal system, especially during infancy, childhood, and adolescence.

Very powerful synthetic forms of vitamin D have been used to treat "renal rickets," a condition caused either by severe kidney disease or by an inherited disorder of renal calcium/phosphorus absorption.

Unsubstantiated claims

Please note that this section reports on claims that have NOT yet been substantiated through scientific studies.

Vitamin D has been purported to be useful in regulating the heartbeat, preventing muscle weakness, and improving arthritis. Research is underway to establish vitamin D's role in the prevention and treatment of cancer, psoriasis, and diabetes.

Recommended intake

Vitamin D is measured in International Units (IU). Currently, one International Unit equals 0.025 micrograms of vitamin D. The RDA is the Recommended Dietary Allowance.

Group

RDA/IU

RDA/mcg

Infants

200 IU

5 mcg

Children

200 IU

5 mcg

Adults (< 51 years)

200 IU

5 mcg

Adults (51 to 70 years)

400 IU

10 mcg

Adults 70 and older

600 IU

15 mcg

Pregnant women

200 IU

5 mcg

Breastfeeding women

200 IU

5 mcg

Note: Adults can meet the requirement for vitamin D without supplementation by exposing their face, hands, arms, or back, without sunscreen, to the sun for 10 to 15 minutes a least two times a week.

People over the age of 50 may be at increased risk of developing vitamin D deficiency. As people age, skin cannot synthesize vitamin D as efficiently and the kidney is less able to convert vitamin D to its active hormone form. Therefore, older adults may benefit from supplemental vitamin D.

People with darker skin are less able to make vitamin D from sunlight so it is important that they get recommended amounts of vitamin D from foods or supplements.

Food source

Nutrient content per 100 grams

Cod liver oil

22,220 IU

Salmon (Atlantic)

650 IU

Salmon (Chinook)

500 IU

Tuna

249 IU

Shrimp

150 IU

Mushrooms

149 IU

Sunflower seeds

92.2 IU

Eggs

49.9 IU

Liver

49.9 IU

Milk, fortified

40 IU

Vitamin D is stable in heat (and therefore does not need to be refrigerated), and is only marginally sensitive to light.

Freezing foods with high vitamin D content (salmon or mackerel) does not reduce their vitamin D content. Vitamin D content generally remains high even when foods are cooked.

When a deficiency of vitamin D occurs during infancy or childhood, the skeletal system develops abnormally, resulting in a condition called rickets. Although it rarely occurs in tropical climates, rickets was common among children in northern cities of the United States and among African American children until milk became fortified with vitamin D supplements. Since vitamin D is necessary to maintain normal bone strength, deficiencies of vitamin D in an adult can lead to loss of calcium and contribute to osteomalacia (softening of bones) as well as osteoporosis (thinning of the bones).

More vitamin D is needed by those who live in subpolar and polar regions where long winter nights cause reduced sun exposure. Melanin, the pigment that gives skin its color, can reduce the skin’s ability to make vitamin D. Individuals with darkly pigmented skin who are unable to get adequate sun exposure and/or consume recommended amounts of vitamin D may need a vitamin D supplement.

Liver diseases such as cirrhosis or renal failure can cause vitamin D deficiencies, as can the ingestion of large amounts of snack foods containing the fat substitutes Olestra or Olean, or the chronic use of mineral oil.

Women who are pregnant or breastfeeding may need to take vitamin supplements, but must consult a physician before doing so.

Deficiency of vitamin D causes rickets in children and osteomalacia or osteoporosis in adults. These conditions result from failure to mineralize new bone. Eventually this leads to demineralization and weakening of existing bone. Similarly, deficiency in infants can lead to craniotabes (soft skull).

Symptoms of rickets include delayed closure of the soft spot (anterior fontanel), often until after the second year of life; macrocephaly (the head may be larger than normal); and abnormalities and defects in the enamel of developing teeth. Other signs are knobby growths at the points where the ribs join the sternum ("rachitic rosary"), thickening of the ankles and wrists, curvature of the spine (either lordosis or scoliosis), bowing of the legs, frequent "greenstick" fractures, muscle weakness, and delayed motor development.

Some of the symptoms of osteomalacia include diffuse bone pain (which can occur anywhere, but frequently occurs in the hips) and muscle weakness.

Some of the symptoms of osteoporosis include back pain, loss of height as the vertebrae become compressed, and fractures that occur with minimal trauma.

Side effects, toxicity, and interactions

Vitamin D in large dosages is toxic. Symptoms in children and adults often develop after several months of excessive use and include constipation, decreased muscle tone (hypotonia), joint pain, irritability, increased thirst (polydipsia), increased urine output (polyuria), loss of appetite, vomiting, and hypertension. Too much vitamin D can also damage the valves in the heart and the kidneys as a result of calcification.

Do not take vitamin D if you have hypercalcemia, signs of vitamin D toxicity, hypervitaminosis D, increased sensitivity to the effects of vitamin D, or decreased kidney capacity.

Use vitamin D with caution if you have arteriosclerosis, hyperphosphatemia, kidney problems, sarcoidosis, or heart problems.